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In an effort to destroy ISIS, beginning in August 2014, the United States, assisted by a handful of other Western and Arab countries, carried out thousands of bombing sorties and cruise missile attacks against ISIS targets in Iraq and Syria. Iraq had consented to the airstrikes in its territory, but Syria had not, and Russia blocked the UN Security Council from authorizing force against ISIS in Syria. The United States invoked several different legal arguments to justify its airstrikes, including the right of humanitarian intervention, the right to use force in a failed state, and the right of hot pursuit, before finally settling on self-defense. Use of force in self-defense has traditionally not been viewed as lawful against non-state actors in a third state unless they are under the effective control of that state but the United States argued that in the aftermath of the 9/11 attacks by al Qaeda, such force can be justified where a government is unable or unwilling to suppress the threat posed by non-state actors operating within its borders. This view was not, however, initially accepted by Russia, China, or even the United Kingdom. But that changed in the aftermath of ISIS attacks against a Russian jetliner and a Paris stadium and concert hall in 2015, leading to the unanimous adoption of a UN Security Council resolution calling on States to use all necessary measures to fight ISIS in Syria without offering a legal basis for military action. This article examines the evolution of the right to use force in self-defense against non-state actors and makes the case that events in 2015 triggered a “Grotian Moment”: a fundamental paradigm shift that will have broad implications for international law.